The Lassen Volcanic Center in northern California is the most recently active group of eruptive centers in a region that has been active for at least the last 3 Ma. Lassen Peak itself was one of only two volcanoes that erupted in the 20th Century in the continental US. Lassen erupted 1914 – 1921 and Mount St Helens in 1980. Note that continental US does not include Alaska, Hawaii, or any of multiple volcanic islands in the Pacific that are US Territories, which is strange nomenclature as Alaska is most certainly part of the North American continent.
Lassen is also one of the few volcanic regions I personally visited, though this trip was at least 60 years ago. While I don’t remember a lot from the trip, I do remember Hot Rock, an enormous chunk of dacite ejected from the crater during the 1915 eruptions that stayed hot for many days following the eruptions. Also remembered Bumpass Hell, though not for any associated hydrothermal action. My parents weren’t all that proud of me repeating the name multiple times. I was fortunate they didn’t invite me to walk home.
The Volcanic Center is surrounded by the Lassen Volcanic National Park created during its eruptive sequence in 1916. The national park combined a pair of neighboring national monuments designated in 1907. Lassen is the southernmost volcano in the Cascades of the western US. The volcanic center is one of the few areas where all four main volcanic forms can be found: stratovolcano, dome, shield, cinder cone. It also has a vigorous hydrothermal system with boiling mud pots, fumaroles and hot springs.
Native Americans lived in the region for thousands of years before European immigrants showed up in the 19th Century. They described the mountain as full of fire and water, and more than a little bit dangerous. Settlers in the fertile Sacramento Valley of Northern California in the 1830s cut trails through the park area. There were unverified reports of fire from the area of Cinder Cone 1850 – 1851. The latest of these was 1859. Actual dating of Cinder Cone activity places the most recent around 1666.
The main park road was built 1925 – 1931. It is not unusual for 12 m of snow to accumulate on the higher portions of the road and last well into July.
A portion of the park was designated as a wilderness area in 1972, which minimizes facilities, signage and trails. A visitor center near Manzanita Lake was closed in 1974 due to proximity to possible landslides from Chaos Crags.
The park is popular averaging 360,000 – 536,000 visitors yearly 2007 – 2016. Recreation is available year-round. Just over 2,300 live within 30 km of the volcano. Nearly 440,000 live within 100 km. Popular activities include climbing, hiking, backpacking, snowshoeing, kayaking, backcountry skiing.
It is located near the northern end of the Sacramento Valley, near the cities of Redding and Susanville. The western part of the park has lava pinnacles, craters and sulfur vents. Glaciers cut canyons and it has multiple lakes and clear streams. The eastern part of the park is a lava plateau with small cinder cones and multiple small lakes.
The park is located at medium to high elevations which influences local climate. Below 2,300 m, the winters are cool to cold and summers are warm. Above this, the winters are harsh and cold and summers cool. Precipitation in the park is high, perhaps the snowiest place in California with over 10 – 18 m of snow measured at various locations. Glaciers eroded the taller volcanoes in the park. Five episodes of Pleistocene glaciation have been identified in the Lassen area.
The greenery is a mixed conifer forest with multiple pine tree species. There are subalpine areas above the tree line with increasing patches of bare rock exposed. Wildlife in the forested areas includes bear, fox, deer, cougar and multiple species of smaller animals.
The volcano is monitored in real time by the USGS. Their Lassen Volcanic Center page has 15 clickable real time seismographs. The park has multiple webcams available through the NPS multimedia web page.
Region
There are multiple volcanic centers in the vicinity of Lassen which have hosted volcanic activity as it has migrated around the region for the last 3 Ma. The Lassen segment of the Cascades arc measures roughly 80 x 80 km.
The oldest of these is the Latour Volcanic Center to the NW of the park. Rocks here are generally over 3 Ma. Its neighboring Yana Volcanic Center is to the SW of Lake Almanor was active 3.4 – 2.4 Ma. These produced volcanic products creating the Tuscan Formation on the west slope of the southernmost Cascade Range. Volcanic debris in the Tuscan includes lahars, erosive debris from glaciation, minor silicic ash flows, airfall tuffs, and several basaltic to andesitic lava flows.
Next oldest is the Dittmar Volcanic Center at the NW end of the Lake Almanor Graben. Eruptive materials here are divided into three groups. Stage 1, early growth of a composite cone 2.3 – 1.6 Ma with thin lava flows and interbedded airfall and volcanoclastic deposits. Stage 2, later growth of the composite cone 1.4 Ma, had thick lava flows with some interlayered explosive or sedimentary deposits (lahars and glacial till). Stage 3 was late silicic volcanism 1.3 Ma with a few lava flows and rhyodacite / rhyolite domes.
There are two satellite dacite dome chains some 40 km SW of Lassen. The Barkley Mountain and Onion Butte group include some 34 dacite domes emplaced on the Tuscan Formation described above. Eruptions here were generally monogenetic. Activity at Onion Butte took place around 2.5 Ma. Activity at Barkley Mountain took place some 1.5 Ma.
The Maidu Volcanic Center appears to have been active at the same time as Dittmar. It is located SW of the park, with its products overlying the Tuscan Formation and the Yana Volcanic Center. It also was active in three stages. Stage 1 wea early composite cone growth with thin lava flows and fragmental deposits erupted from a central vent. Stage 2 was later composite cone growth with thin dacite lava flows erupted from flank vents. Stage 3 was late silicic volcanism that erupted five large, thick rhyodacite / rhyolite lava flows. These are some of the largest silicic lava flows in the Cascades, with a volume around 35 km3.
Lassen
Lassen is the most recent volcanic complex in the region. Its major divisions include the Rockland caldera complex, Brokeoff Volcano and the Lassen dome field. These divisions erupted in separate stages from the same magmatic system.
The Rockland Caldera Complex produced the Rockland tephra and a group of dacite to rhyolite domes and lava flows 825 – 609 ka. Initial activity was effusive with lava flows and dome extrusion. It ended explosively with the Rockland tephra 610 ka.
Brokeoff Volcano (Mount Tehama) is a large (80 km3) composite volcano that began erupting almost immediately after the formation of the Rockland Caldera in 610 ka. The volcano filled the newly formed caldera with two sequences of eruptive products. The Mill Canyon sequence was dozens of small-volume basaltic andesite to dacite lava flows interlaid with layers of explosive deposits all erupted from a central vent 590 – 470 ka. The Diller sequence is 6 thick, large volume similar lava flows erupted from flank vents 470 – 385 ka.
Activity changed dramatically from andesitic stratocone to the Lassen domefield 385 – 315 ka. This is a core of dacite domes surrounded by an arc of andesitic lava flows. The dacite domes erupted on the northern flank of Brokeoff Volcano. The early group formed 15 dacite domes, the 310 – 240 ka Bumpass. The later formed 7 dacite / rhyodacite domes, the 70 ka – present Eagle Peak sequence. The andesitic units erupted 315 – 240 ka and 90 ka – present, the older and younger Twin Lakes sequence. There was no volcanism in the Lassen Volcanic Center 190- 90 ka.
The domefield and andesitic eruptive products were formed by magma mixing. Eruptive units from this are variable in appearance and composition. The Eagle Peak sequence of domes also included lava flows and pyroclastic deposits erupted on the northern part of the Lassen domefield. Eagle Peak sequence has prominent young volcanic features of Lassen Peak (27 ka), Chaos Crags (1.1 ka) as well as the products of the previous five eruptions. The Twin lakes sequence includes andesitic and basaltic andesitic lava flows and cones erupted around the edge of the Lassen domefield, primarily on the central plateau of the park. The two youngest eruptions in the park, Cinder Cone and the 1914 – 1917 eruption of Lassen Peak are both parts of the Twin Lakes sequence.
Lassen Peak is an enormous rhyodacite lava dome that first started forming some 27 ka. Initial dome growth underwent significant glacial erosion 25 – 18 ka. There is a bowl-shaped depression in the NE flank of the dome eroded by a glacier that stretched some 11 from the source. By 18 ka, the dome grew through the lavas of Tehama’s northern flank, destroying it. It shattered overlying rock, creating a blanket of talus around the steep sides of the emerging dome. It quickly reached its current height.
Over the last 1,000 years, activity has produced six lava domes, erupted tephras and pyroclastic flows, and built the Cinder Cone and Fantastic Lava Beds. It also created rockfalls at Chaos Jumbles.
Lassen Peak is the only Cascade volcano that tops out above 3,000 m that is not a stratovolcano. It is one of the largest lava domes on earth, over 610 m above its surroundings. The volcanic field surrounding Lassen behaves like a monogenetic volcanic field which erupts from multiple locations but never from the same vent twice. Glacial erosion has covered the dome with broken rock fragments, talus. The crag formations on the southern flank near the summit have not been significantly eroded.
The Lassen Volcanic Center includes Brokeoff Mountain, the Lassen Peak lava dome, a number of small andesitic shield volcanoes, and at least 30 dacitic lava domes. The hydrothermal system inside the Volcanic Center is the largest geothermal field in the continental US outside of Yellowstone.
Chaos Crags are five small lava domes, the youngest dome field in the center. They are as tall as 550 m above their surroundings. Eruptions that built them were vigorous explosive activity producing pumice and ash followed by effusive activity. These created unstable domes, which collapsed from time to time creating pyroclastic flows. Six domes were initially grown. One of them was destroyed by collapse and subsequent pyroclastic flow. Roughly 350 years ago, one of the domes collapsed creating Chaos Jumbles from at least three rockfalls that traveled over 6 km from the dome.
Cinder Cone is the youngest structure in the center. It is surrounded by block lava with craters on its summit. It produced at least 5 andesitic lava flows, two scoria cones, the first of which was destroyed by lava flows from its base. There were reports of explosive activity and lava flows running down its sides in 1850 – 1851. But these reports were not supported by dating or the presence of willow growing near the summit crater in 1850 still present 30 years later.
The Caribou Volcanic Field is located some 20 – 30 km E of Lassen Peak. It is monogenetic cinder cones, lava flows, and small to medium cones. It was active 425 ka – present. This field appears to be a new volcanic center not yet sufficiently vigorous to trigger crustal melting and silicic volcanism. Its vents are generally grouped in arrays controlled by subsurface faults that provided pathways for rising magma.
Hydrothermal system
No discussion about Lassen would be complete without considering the extensive hydrothermal system associated with the volcano. This is the best exposed and largest hydrothermal system in the Cascades, accounting for a full third of total high temperature hydrothermal heat discharge in the US Cascades (140 of 400 MW thermal). The heat is supplied by crystallization and cooling of silicic magma at a depth of 4-5 km below the volcano. This depth is defined by clustering of observed seismicity.
There are multiple zones associated with this system. External (meteoric) water appears to enter the system from the north side of Lassen Peak. It moves to depth where it encounters near plastic silicic magma where is it boiled under pressure. It rises to the boiling zone, a liquid dominated zone. As it continues to rise, rock pressure decreases to the point where the water is vaporized in the vapor dominated zone. Its final journal is through cracks in a hard cap called the condensate zone to exit the system mostly in the vicinity of Bumpass Hell, the most prolific hydrothermal zone in the park. Gas depleted thermal water flows farther south to exit in the vicinity of Growler and Morgan Hot Springs on the southern flank of the system.
All the thermal features in the park are driven by boiling underground hot water. The hottest and most vigorous features are at Bumpass Hell, the principal outflow area of the system. High temperature steam jetting from Big Boiler, the largest fumarole in the park is as high as 161° C, one of the hottest hydrothermal fumaroles in the world. Most of the features contain mixtures of steam and near surface ground water with temperatures near local boiling point of water. These waters are typically acidic and with one exception are not safe for bathing.
These systems vary in output and temperature seasonally and from year to year, generally cooler in the spring and drier and hotter in later summer. As with other hydrothermal systems, local features shift position, start, stop, and evolve into different features over time. There is only one geyser in the park. It is not considered to be a true geyser, being caused by cool surface stream interacting with an active fumarole. There are at least 9 active thermal areas, most of them on the southern part of the National Park.
Eruptions
The 1914 – 1917 series of eruptions is the most recent explosive activity from Lassen. At least one source extends this series through 1921.
Activity began May 30, 1914 with a steam explosion that created a small crater with a deep lake at the summit. The crater grew over the next 11 months via a series of 180 additional phreatic explosions, eventually 300 m wide. May 14, 1915 the volcano erupted lava blocks and an ash plume that reached as far as 32 km W of the volcano. An effusive eruption took place following this explosion, filling the crater with a new dacitic lava dome. This dome was destroyed four days later with an explosive eruption creating a new crater at the summit. At the time of this explosion, there was over 9 m of snow on the mountain, so the explosive debris created a lahar that traveled up to 65 km from the summit. Most of the travel was down surrounding river valleys. Although nobody was killed, the lahar did cause minor injuries, destroy homes near the river, and kill fish in the river. Effusive dacite also replaced the destroyed dome and filled the newly formed crater once again.
The largest explosion took place May 22, producing a plume topping out over 9 km. The plume was visible some 240 km to the west. When the plume collapsed, it created a pyroclastic flow that destroyed nearly 8 km2 around the summit. The eruption also created additional lahars. Ash fell some 40 km NE of the volcano, with detectible ash as far as 450 km E at Elko, NV.
While the eruptive volume was relatively small estimated at 0.03 km3, nearly 8 km2 of vegetation on the volcano’s NE flank was destroyed. This is today called the Devastated Area.
Phreatic explosions continued for several years following 1915. A strong explosion May 1917 created the Northern Crater on Lassen’s summit. This also produced an ash plume pushing 4 km above the summit. There were at least 21 additional phreatic explosions through June 1917. Additional explosions took place June and Oct 1919, April 1920, with the last one Feb 1921. Eruptions following 1917 are blamed on spring snowmelt percolating into Lassen triggering steam explosions.
Fumaroles have been active through the 1950s but have weakened over time. Climbers reported steam eruptions in the summit craters for decades after the last phreatic explosions in 1921.
Basaltic lava flows are the most common activity in the Lassen Volcanic Center. They can also produce more violent and hazardous silicic lava flows and dacite domes. These eruptions built unstable domes that have collapsed producing pyroclastic flows and debris avalanches that can travel for kilometers from the volcano. Due to the significant snowfall yearly on the region, a poorly timed eruption can produce lahars that have traveled over 60 km from the volcano in the past. Today, rockfalls still pose a smaller but more common hazard to visitors.
Lassen is one of four Cascade volcanoes that has undergone subsidence. Lassen’s subsidence took place within a circle some 20 km in diameter centered on the volcano.
There was a Smithsonian GVP Bulletin Report Jun 1992 (BGVN 17:06) that reported a seismicity increase at several volcanic centers in California following a M 7.5 earthquake 200 km E of Los Angeles. Small events were noted in the 24 hours following the quake at Lassen, Shasta, Medicine Lake and the Geysers. Lassen had the strongest response centered 3 km SW Lassen Peak.
Tectonics
Tectonics of northern California and the Lassen Park have changed in recent millennia. Volcanism in the Cascades was originally driven by the subduction of the Juan de Fuca, Gorda and Explorer Plates beneath the North American Plate. North of California, the subduction of the Juan de Fuca Plate supplies melt to power the Cascades volcanic arc. These smaller plates are the unsubducted remnants of the long gone Farallon Plate.
In California, things became a bit complex, with the arrival of the East Pacific Rise south of California splitting off Baja California. The East Pacific Rise, a spreading center that forms the boundary between the previously subducting Farallon Plate and the Pacific Plate, followed the Farallon Plate, which was being subducted under North America. Motion of the Pacific Plate was and is generally to the NNW. Following the arrival of the East Pacific Rise, tectonic motion transitioned from an easterly subduction collision to a more northerly strike – slip movement, creating both the San Andreas Fault system on the coast and the Walker Lane on the California – Nevada border. We discussed the tectonics associated with the Walker Lane in a 2018 post. The new strike-slip motion started peeling Baja California off North America some 12 Ma.
The San Andreas and Walker Lane systems started progressive rifting along the length of California some 13 – 6 Ma. About 75% of the motion takes place along the San Andreas. The rest of it along the still-forming Walker Lane. The area between the two fault systems contains multiple blocks, one of which is the Sierra Nevada mountains. Lassen sits at the northern end of the Sierra Nevada block (or microplate) as faulting associated with the Walker Lane starts its turn westerly toward the California and Oregon coastlines. Lassen is also located at the southern end of the present-day Cascades Arc.
The final piece of the puzzle is the Range and Basin Province of North America. The Walker Lane separates the spreading of the Basin and Range Province from the Sierra Nevada block / microplate. At this point, it is difficult to say which is responsible for associated rifting that allows magma to reach the surface: Walker Lane rifting or Basin and Range rifting? Either way, it appears rifting is involved and new magma should be available.
Conclusions
Lassen is one of those volcanic systems with more published information than you know what to do with, making writing this post more about trying not to be buried in available information than finding it. That being said, it continues to be an active volcanic system, with a known magma body some 4-5 km below the surface. This magma body powers the second most vigorous hydrothermal system in the contiguous US. While it does not erupt often, it gets sufficient recharges of new basalt to trigger new eruptions. Lassen has been active for over 3 Ma. Given the ongoing rifting associated with the Walker Lane and the Basin and Range Province, it does not appear to be finished yet.
Additional information
Smithsonian GVP – Lassen Volcanic Center
Eruption History of the Lassen Volcanic Center and the Surrounding Region, USGS
Lassen Volcanic Center, Home, USGS
The Eruption of Lassen Peak, NPS
Four days in May: Mount Lassen erupted 104 years ago, A Alden, KQED, May 2019
Time-scale and mechanism of subsidence at Lassen Volcanic Center, CA from InSar, Parker, et al, 2016
Everything you need to know about Lassen Volcanic National Park, Culture Trip
The Lassen hydrothermal system, Ingebritsen, et al, Feb 2016
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